Retaining walls: segmental block and quarried boulder

Segmental block is modular, stacks with a built-in setback, and suits straight runs, terraces and tight corners where the line has to be exact. Quarried sandstone block and boulders get set one at a time with an excavator or a track loader, keyed into each other and into the slope, weathered face out and buried about a third of the way in. Boulder walls read as part of the ground instead of as a product. Block goes taller in a smaller footprint and holds a radius. Which one fits comes down to the slope and to what a machine can reach.

A retaining wall is only as straight as its first course. That course sits on 3/4" clean crushed base, 8" to 16" deep depending on the soil and the application, compacted in lifts, and then it gets buried. The buried course counts toward the wall height even though nobody sees it, and the maximum height a block system is rated for as a gravity wall includes it. We level that first course block by block. Any error left in it shows up multiplied five courses up, and there is no fixing it from the top.

Walls fail when they end up holding water instead of soil. Saturated backfill weighs more and pushes harder, and here it also freezes and expands behind the block. So behind every wall we build goes clean drainage rock and a perforated drain line, daylighted out to somewhere the water can actually go. Daylighted is the part that gets skipped. A drain line that dead ends behind a wall is a pipe full of water. Where there is no low point to run to, the proposal says how the water gets out.

Backfill goes in and gets compacted in lifts as the wall goes up, course by course, not dumped in at the end. Compacting behind a finished wall either does not happen or pushes the wall out of line. Geogrid is a mesh laid back into the soil between courses, tying the block and the ground behind it into one mass. We lay it in when the height calls for it, when there is a slope above, or when there is a surcharge on top: a driveway, a patio, a shed. Those loads take a gravity wall over.

Under the 2021 residential code Montana has adopted, a retaining wall does not need a building permit if it is four feet or less measured from the bottom of the footing to the top of the wall, with nothing surcharging it. Taller than that, or carrying a driveway or a structure, and it needs a permit and an engineered design. The City of Bozeman and Gallatin County run that differently, and county addresses bring wells and septic drain fields into where you can dig. We say at the site visit which one you are in.

Most walls we build exist because someone needs flat ground on a lot that does not have any. Hillside lots in Big Sky and out along the benches get terraced so each run stays short, usually with steps between the levels. In newer subdivisions the grade change lands at a property line or a drainage swale, and a swale cannot be filled, so the wall works around it. Older Bozeman neighborhoods bring mature trees and narrow access, and we would rather move the wall line than cut the roots holding a tree up. The wall gets proposed with the rest of the hardscape.